Compound-Specific Chlorine Isotope Analysis: A Comparison of Gas Chromatography/Isotope Ratio Mass Spectrometry and Gas Chromatography/Quadrupole Mass Spectrometry Methods in an Interlaboratory Study

Abstract

Chlorine isotope analysis of chlorinated hydrocarbons like trichloroethylene (TCE) is of emerging demand because these species are important environmental pollutants. Continuous flow analysis of noncombusted TCE molecules, either by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) or by GC/quadrupole mass spectrometry (GC/qMS), was recently brought forward as innovative analytical solution. Despite early implementations, a benchmark for routine applications has been missing. This study systematically compared the performance of GC/qMS versus GC/IRMS in six laboratories involving eight different instruments (GC/IRMS, Isoprime and Thermo MAT-253; GC/qMS, Agilent 5973N, two Agilent 5975C, two Thermo DSQII, and one Thermo DSQI). Calibrations of 37 Cl/ 35 Cl instrument data against the international SMOC scale (Standard Mean Ocean Chloride) deviated between instruments and over time. Therefore, at least two calibration standards are required to obtain true differences between samples. Amount dependency of δ 37 Cl was pronounced for some instruments, but could be eliminated by corrections, or by adjusting amplitudes of standards and samples. Precision decreased in the order GC/IRMS (1σ ≈ 0.1%), to GC/qMS (1σ ≈ 0.2À0.5% for Agilent GC/qMS and 1σ ≈ 0.2À0.9% for Thermo GC/qMS). Nonetheless, δ 37 Cl values between laboratories showed good agreement when the same external standards were used. These results lend confidence to the methods and may serve as a benchmark for future applications.

Current challenges in compound-specific stable isotope analysis of environmental organic contaminants

“…In commercial continuous-flow IRMS, in contrast, CO 2 monitoring gas bypasses the chromatographic system so that errors from chromatography or incomplete combustion are not accounted for, and the amplitudes of sample and reference gas are typically not the same. Such "monitoring gases" are useful to monitor the performance of the mass spectrometer and also to enable crude isotopic calibration of analyte peaks, but they cannot be used to achieve a reliable isotopic calibration on the basis of the "principle of identical e For trichloroethylene (TCE) with GC-quadrupole MS [6] treatment of sample and reference material". The use of a single monitoring gas for calibration also precludes any two-point calibration approach based on two isotopically different reference materials [8,9].…”

“…4 upper panel) [99] or a quadrupole MS [100][101][102]. Instead of species that contain only one chlorine atom (e.g., 37 [101,102], fragment ions [6,99], or a weighted combination of both [100]. The applicability of the isotopologue approach has been theoretically validated [103] and practically investigated [102].…”

“…Currently, the development of compound-specific chlorine isotope analysis is at a stage at which different approaches have been proposed, but it is unclear how their precision and trueness compare in real-world applications. A recent method comparison in an interlaboratory study [6] has taken a step toward such a validation for trichloroethylene as example and has shown that precise and consistent isotope values can be obtained by different laboratories by use of GC-IRMS or GC-quadrupole MS. At the same time, the study emphasizes the need for method calibration with at least two different isotope standards which are chemically identical to the compound to be analyzed, yet sufficiently different in 37 Cl abundance (see Part 7).…”

Carbon and chlorine isotopologue fractionation of chlorinated hydrocarbons during diffusion in water and low permeability sediments

“…Calibration with external standards was performed to receive delta values on the SMOC scale. A recent inter laboratory comparison for different GC-qMS equipment, demonstrated that a lack of calibration of raw GC-qMS chlorine isotope ratios potentially leads to variation of the difference in isotope ratios between two samples by as much as a factor of 1.3 (Bernstein et al, 2011).…”

Unravelling long-term source removal effects and chlorinated methanes natural attenuation processes by C and Cl stable isotopic patterns at a complex field site

“…Changes in both carbon and chlorine isotope values in the field should be N2‰ so that the degradation is considered significant (Hunkeler et al, 2008;Bernstein et al, 2011).…”